Embodiments of the described invention relate to the field of airborne geological mapping and receiver systems used for such mapping.
Active source electromagnetic surveying such as time domain electromagnetic (TDEM) surveying is a rapidly developing area of geophysical surveying. It encompasses ground based and airborne applications. TDEM geological mapping involves measuring the magnetic response of the earth to a primary magnetic field transmitted by the survey system. The relation between the transmitted field and the response is used to calculate the electrical resistivity structure of the earth, from which geological information is inferred.
An example of a TDEM surveying system and method is shown in U.S. Pat. No. 7,157,914.
Natural source electromagnetic surveying has traditionally been performed using ground based techniques such as the magnetotelluric (MT) technique. Recently, airborne surveying using natural source electromagnetic techniques has become practical. In these techniques, two or more components of naturally occurring random fluctuations of the electromagnetic field of the earth are measured (possibly at different locations), and the frequency dependent transfer functions between the measured components are calculated. As in active source methods, the transfer functions are interpreted in terms of the electrical resistivity structure of the earth, from which geological information is inferred.
An example of a natural source electromagnetic surveying system is shown in U.S. Pat. No. 6,876,202
An active source electromagnetic survey system has a transmitter and a receiver, while a natural source system has only a receiver. Typically a transmitter includes a coil of one or more turns of electrical wire. When an electric current is passed through the wire, a magnetic field is transmitted. In TDEM surveying, a pulsed current of alternating polarity is used, and the response of the earth is sensed in the “off” time between transmitter current pulses.
A receiver or sensor typically includes of one or more multiturn coils of electrical wire. In the presence of a changing magnetic field, an electrical voltage appears across the terminals of each coil. This voltage can be amplified and recorded. Coils may have different orientations, making them sensitive to variations in magnetic field components having different directions. Other things being equal, the sensitivity and noise floor of a receiver coil improve as the coil is scaled up in size. The signals used in natural source systems are typically weaker, requiring larger receiver coils, compared to active source systems.
The response to movement and vibration of receivers used in active and natural source surveying systems is a significant noise source, especially in a turbulent airborne environment, becoming increasingly important as the signal frequency decreases below 100 Hz. A major contribution to this type of noise is caused by the motion of the receiver coil(s) relative to the static geomagnetic field. Motion or vibration that changes the total geomagnetic flux passing through a receiver coil causes a electrical voltage to appear across the terminals of the coil. In the case of a rigid receiver coil, this can be caused by rotation of the coil. No receiver coil is perfectly rigid, so flexing of the coil also contributes to such voltages. These voltages are a type of noise that interferes with the desired signal. Techniques for reduction of noise are important.
In some electromagnetic survey systems, the receiver is sensitive to changes in one component of the magnetic field, typically a nominally vertical component. Receivers that independently measure changes in two or three substantially orthogonal components of the magnetic field provide improved geological information, but are bulkier than single axis receivers.
Improved receiver systems for airborne geophysical survey systems are desirable.